Caption: Johann Joachim Becher (1635-1682), German alchemist and physician. Becher was a self-educated minister who was appointed professor of medicine at the University of Mainz in 1657. He published a number of books, including the Physica Subterranea (1667) in which he initiated the idea of phlogiston, later posited by his student G.E. Stahl. The theory holds that flammable materials release the colourless, odourless, tasteless and weightless substance phlogiston when they are burned. Antoine-Laurent Lavoisier later disproved the phlogiston theory.